424 informed and consented subjects undergoing thoracic transplantation or left ventricular assist device (LVAD) implantation under the care of one or more investigators will be prospectively enrolled over a three-year period (one-year for follow-up). Patients will be randomly assigned 50/50 according to randomization strata to one of two standard of care pulmonary vasodilation therapy, iNO vs iEPO. Additional study procedures will involve data collection and blood sampling.